The Plant Vascular System: Evolution, Development and FunctionsF

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Table 3. Transporters involved in vascular loading and unloading of cationic microelements Insights into Plant Vascular Biology 355 Family Gene Microelement Function Reference YSL AtYSL1 Fe-NA Seed loading, phloem loading Le Jean et al. 2005 AtYSL2 Fe, Zn Xylem loading/unloading DiDonato et al. 2004 Schaaf et al. 2005 AtYSL3 Fe, Cu, Zn Phloem loading Waters et al. 2006 OsYSL2 Fe-NA Mn-NA Phloem loading/unloading to grains Koike et al. 2004 OsYSL18 Fe(III)-DMA Phloem loading/unloading to reproductive organs Aoyama et al. 2009 TcYSL3 Fe-NA, Ni-NA Xylem unloading Gendre et al. 2007 TcTSL7 Ni-NA Xylem unloading Gendre et al. 2007 OPT AtOPT3 Mn Fe Seed loading Stacey et al. 2008 Ferroportin IREG1/FPN1 Fe Xylem loading Morrissey et al. 2009 MATE FRD3 Citrate Xylem loading Durrett et al. 2007 Yokosho et al. 2009 Yokosho et al. 2010 P-type ATPases AtHMA5 Cu Xylem loading André-Colás et al. 2006 Kobayashi et al. 2008 AtHMA2/4 Zn Xylem loading Hussain et al. 2004 Mills et al. 2005 Wong and Cobbett 2009 Verret et al. 2004, 2005 AhHMA4 Zn Xylem loading Hanikenne et al. 2008 transported by FPN1 has yet to be established (Morrissey et al. 2009). The Arabidopsis P-type ATPases, AtHMA5 and AtHMA2/4, have been implicated in Cu and Zn efflux, respectively, into the xylem at the root level, for long-distance transport to the shoots (Hussain et al. 2004; Mills et al. 2005; Andrés- Colas et al. 2006). Consistent with this model, both hma5 and hma2hma4 loss-of-function mutants accumulate increased levels of the corresponding metal within the root, and show lower levels in their shoots (Hanikenne et al. 2008; Wong and Cobbett 2009). HMA5 is predominantly expressed in the root and is specifically induced by excess Cu. Mutants of HMA5 overaccumulate Cu in the root, suggesting a compromised efflux system. Further evidence in support of the role of HMA5 in xylem transport of Cu from the roots to the shoots comes from a study of natural variation in Cu tolerance among Arabidopsis accessions, which identified HMA5 as a major QTL associated with Cu translocation capacity and sensitivity (Kobayashi et al. 2008). HMA2 and 4 are present in the plasma membrane of root and shoot vascular tissues (Mills et al. 2003; Hussain et al. 2004; Verret et al. 2004; Mills et al. 2005; Verret et al. 2005; Williams and Mills 2005; Sinclair et al. 2007; Blindauer and Schmid 2010). In addition, functional analysis of HMA4 in A. halleri and A. thaliana showed that silencing of AhHMA4, by RNA interference, completely suppressed Zn hyperaccumulation. These studies provided a clear demonstration that HMA4 plays a key role in xylem loading and, consequently, in root-to-shoot transport of Zn (Hanikenne et al. 2008). Organic acids may also have a role in xylem Fe loading. Citrate has been described as an Fe(III) chelator in the xylem sap (Rellán-Álvarez et al. 2010) and FRD3 (Ferric Reductase Defective), a transporter of the MATE family, is localized to the plasma membrane of the pericycle and vascular cylinder. FRD3 proteins facilitate citrate efflux into the xylem of the root vasculature and have been described in Arabidopsis (Durrett et al. 2007), rice (Yokosho et al. 2009) and rye (Yokosho et al. 2010). Mutant frd3 plants are chlorotic, show reduced citrate and Fe concentrations in the xylem and the shoot, accumulate Fe in the root, and exhibit constitutive expression of the Fe uptake components, thus suggesting that FRD3 is necessary for efficient Fe transport to the shoot through the transpiration stream. Also, independent Fe-citrate and Fe-NA xylem loading systems may complement each other, as in the frd3 mutant, the nicotianamine synthase NAS4 gene is induced, and the double mutant nas4x-2/frd3 shows impaired growth and low Fe levels in the shoot (Schuler et al. 2010). FRD3 is constitutively expressed in the hyperaccumulators A. halleri and N. caerulescens compared to A. thaliana and N. arvensis, and may also play a role in Zn transport (Talke et al. 2006; van de Mortel et al. 2006). However, this overexpression may be related to an altered Fe homeostasis leading to high Zn concentrations in the hyperaccumulators (Roschzttardtz et al. 2011).
356 Journal of Integrative Plant Biology Vol. 55 No. 4 2013 Vascular loading and unloading of anionic micronutrients Xylem loading of B is mediated by BOR1 (Takano et al. 2001; Miwa and Fujiwara 2010). BOR1 is an anion efflux system that is strongly expressed in the root pericycle cells surrounding the xylem vessels. The bor1-1 mutant is defective in xylem loading of B (Takano et al. 2002). The specific chemical form of the substrate for BOR1 remains unknown, but electrophysiological analyses in the human homolog, NaBC1, suggest borate anion as the likely candidate (Park et al. 2004). There are six BOR1 paralogs in the A. thaliana genome which may also have roles in xylem-phloem loading and unloading (Miwa and Fujiwara 2010). A second B transporter, NIP6,1, is a channel protein required for proper distribution of boric acid, particularly to young developing shoot tissues (Tanaka et al. 2008). This transporter is predominantly expressed in nodal regions of shoots, especially the phloem region of vascular tissues, where it is likely involved in xylem-phloem transfer of boric acid. The mechanisms for Mo loading and unloading in the vascular tissues remain to be elucidated. To date, only one Mo transporter from Arabidopsis, MOT1, has been identified in plants. MOT1 is a high-affinity molybdate transporter localized to the plasma membrane or mitochondrial membranes, and plays an important role in efficient Mo uptake from soils and accumulation within the plant (Tomatsu et al. 2007; Baxter et al. 2008). MOT1 belongs to the family of sulphate transporters, SULTR (Hawkesford 2003), which has 14 members in A. thaliana. It is tempting to speculate that some of these transporters may be involved in vascular tissue loading or unloading. Systemic Signaling: Pathogen Resistance Like all living organisms, plants have to constantly resist pathogenic microbes. The absence of a circulatory vascular system and their sessile nature can pose particular problems. Plants have therefore evolved unique defense mechanisms to ensure survival. The multiple modes of plant defense include both passive and active mechanisms that provide defense against a wide variety of pathogens. Active defense includes the production of antimicrobial compounds, cell wall reinforcement via the synthesis of lignin and callose, and the specific induction of elaborate defense signaling pathways. These include species level (non-host) resistance, race-specific resistance expressed both locally and systemically, and basal resistance. Race-specific resistance is induced when strain-specific avirulent (Avr) proteins from the pathogen associate directly/indirectly with cognate plant resistance (R) proteins (reviewed in Jones and Dangl 2006; Caplan et al. 2008). Induction of R-mediated signaling is often accompanied by the onset of a hypersensitive response (HR), a form of PCD resulting in necrotic lesions, at the site of pathogen entry (Dangl et al. 1996). HR is one of the first visible manifestations of pathogeninduced host defenses, and is thought to help confine the pathogen to the dead cells. R-mediated signaling is also often accompanied by the induction of a robust form of resistance against secondary pathogens in the systemic parts of the plants, termed systemic acquired resistance (SAR) (Durrant and Dong 2004; Vlot et al. 2008; Spoel and Dong 2012). Identified as a form of plant immunity nearly 100 years ago, SAR is a highly desirable form of resistance that protects against a broad-spectrum of pathogens. SAR involves the generation of a mobile signal at the site of primary infection, which moves to and arms distal portions of a plant against subsequent secondary infections (Figure 27). The identification of this signal could greatly facilitate the use of SAR in protecting agriculturally important plants against a wide range of pathogens. Because of its unique mechanistic properties and its exciting potential applications in developing sustainable crop protection strategies, SAR has been one of the most intensely researched areas of plant biology. The last decade has witnessed considerable progress, and a number of signals contributing to SAR have been isolated and characterized. Despite concerted efforts to harness this mode of plant immunity, the plant defense field lacks a consensus regarding the identity of the SAR signal, whether this signal constitutes multiple molecular components, and how these component(s) might coordinate the systemic induction of broad-spectrum resistance. Among the signals contributing to SAR are salicylic acid (SA) and several components that feed into the SA pathway, including the methylated derivative of SA (MeSA; Park et al. 2007), the diterpenoid dehdryoabietinal (DA; Chaturvedi et al. 2012), the nine carbon (C9) dicarboxylic acid azelaic acid (AA; Jung et al. 2009), auxin (Truman et al. 2010), the phosphorylated sugar glycerol-3-phosphate (G3P; Chanda et al. 2011; Mandal et al. 2011), and two lipid transfer proteins (LTPs), Defective in Induced Resistance (DIR1; Maldonado et al. 2002) and AA insensitive (AZI1; Jung et al. 2009). Jasmonic acid (JA) has also been suggested to participate in SAR (Truman et al. 2007), but its precise role remains contentious (Attaran et al. 2009). The diverse chemical natures of the SAR-inducing molecules have led to the growing belief that SAR might involve the interplay of multiple diverse and independent signals. In this final section of the review, we will evaluate the role of SA and the recently identified mobile inducers of SAR. SA and SAR SA is a central and critical component of SAR. The biosynthesis of SA occurs via the shikimic acid pathway, which bifurcates into two branches after the biosynthesis of chorismic acid. In one branch, chorismic acid is converted to SA via
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